1. Field of the Invention
The present invention relates generally to providing velocity feedback in an actuator assembly. More particularly, the present invention relates to a method and apparatus for providing velocity feedback to compensate for disturbance torque.
2. Description of the Related Art
Nearly all servers and computers use disc drives to store mass quantities of digital information. Generally, a disc drive includes a disc (often referred to as a platter) and a read/write device. The disc is commonly a hard disc, but also may be a soft or flexible disc. Typically, the disc includes a surface of magnetic film to store data and is supported and spun by a spindle motor. During disc drive operation, the data on the disc is accessed by the read/write device, which is commonly referred to as a head.
To enable the head to access different sections, the disc is rotated under the head, which is supported and manipulated by an actuator assembly. The rotation of the disc creates a curtain of air, commonly referred to as an air bearing, which supports the head and prevents physical contact with the disc surface. When the disc drive is not in use, the head is unloaded from the disc and moved across a ramp assembly to be safely stored. When the drive is started, the actuator assembly moves the head back across the ramp to load it onto the disc.
Because of the growing popularity and demand for portable and wireless devices, one area of disc drive technology that has become extremely important is the reduction of size or form factor of the drive. For designers of small form factor disc drives, one of the most difficult challenges is to adapt the drives for use in applications, in which an external power supply is absent. In contrast to typical desktop systems, in which disc drives are usually operating and spinning constantly, disc drives are preferably turned off when not in use to minimize power consumption in portable devices.
Unfortunately, increasing the number of times that a disc drive is turned on and off necessarily increases both the number of loading and unloading operations and the chance that the head assembly will be damaged. During disc drive operation, the actuator assembly must withstand and compensate for a variety of changing forces or torques acting on the actuator arm to prevent such a collision. In particular, during unloading and loading, it is critical that the velocity of the actuator arm must be as smooth as possible.
A conventional method of velocity control is accomplished by obtaining velocity feedback from the actuator arm. The method measures a back electromagnetic force (BEMF) that is generated by a voice coil motor (VCM) of the actuator arm. The BEMF provides velocity information for the actuator arm and is feedback to a controller, which adjusts a velocity command signal output to the actuator arm to move the arm at the desired speed.
Because the margin of error for controlling the speed of the actuator arm to prevent damage to the disc drive shrinks as the form factor of disc drives decreases, ramp load and unload technology must continue to improve to keep the velocity of the actuator arm as smooth as possible and prevent damage to the disc drive. In view of the foregoing, it is desirable to have a method and apparatus for providing velocity feedback that compensates for external torque disturbances.